Two highly inbred MHC-identical lines of chickens differ markedly in susceptibility to viral oncogenesis, and express contrasting T cell surface antigens determined by the independent Ly-4 and Th-1 loci. These antigens may be markers for partially overlapping subsets of T cells, and there is evidence that one of these loci, and perhaps both, are associated with resistance to Marek's disease oncogenesis. The extent to which these surface antigens are markers for distinct subsets of central and peripheral T lymphocytes will be analyzed during embryogenesis and in growing chickens, using immunofluorescent techniques and double labeling. In vitro methods will be used to investigate whether the expression of suppressor and cytotoxic activity of immunocompetent cells is associated with the expression of inherited resistance to viral oncogenesis in Marek's disease, and attempts will be made to relate these functional properties to the T cell subsets.